[es6] Move builtin constructors for primitives to strict mode.

The ES6 specification says that "Built-in functions that are ECMAScript
function objects must be strict mode functions", which in particular
means that you can never test for them using the "caller" field of a
sloppy mode function.

CQ_INCLUDE_TRYBOTS=tryserver.v8:v8_linux_layout_dbg,v8_linux_nosnap_dbg
R=mstarzinger@chromium.org
BUG=v8:105
LOG=n

Review URL: https://codereview.chromium.org/1347663002

Cr-Commit-Position: refs/heads/master@{#30750}

src/string.js

@@ -36,6 +36,8 @@ utils.Import(function(from) {
 //-------------------------------------------------------------------
 
 function StringConstructor(x) {
+  // TODO(bmeurer): Move this to toplevel.
+  "use strict";
   if (%_ArgumentsLength() == 0) x = '';
   if (%_IsConstructCall()) {
     %_SetValueOf(this, TO_STRING_INLINE(x));

src/v8natives.js

@@ -1341,6 +1341,8 @@ utils.InstallFunctions(GlobalObject, DONT_ENUM, [
 // Boolean
 
 function BooleanConstructor(x) {
+  // TODO(bmeurer): Move this to toplevel.
+  "use strict";
   if (%_IsConstructCall()) {
     %_SetValueOf(this, ToBoolean(x));
   } else {
@@ -1390,6 +1392,8 @@ utils.InstallFunctions(GlobalBoolean.prototype, DONT_ENUM, [
 // Number
 
 function NumberConstructor(x) {
+  // TODO(bmeurer): Move this to toplevel.
+  "use strict";
   var value = %_ArgumentsLength() == 0 ? 0 : ToNumber(x);
   if (%_IsConstructCall()) {
     %_SetValueOf(this, value);

test/mjsunit/regress/regress-105.js

@@ -26,12 +26,12 @@
 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 var custom_valueOf = function() {
-  assertEquals(Number, custom_valueOf.caller);
+  assertEquals(null, custom_valueOf.caller);
   return 2;
 }
 
 var custom_toString = function() {
-  assertEquals(String, custom_toString.caller);
+  assertEquals(null, custom_toString.caller);
   return "I used to be an adventurer like you";
 }